Between the gun safes and the fire safes you can buy at Home Depot and such, ISTM that SDBs are really a 1930s solution to 1930s problems.
Back in the day when people had truly irreplaceable papers and things like bearer bonds were out there, I could see ordinary middle to upper-middle class folks needing bank-level security for some of their stuff.
But there is no important paper I own that isn’t just a copy of a record in a computer someplace. And all of what would have been “negotiable instruments” 50 years ago are now just entries in the computers at Vanguard or whoever. Despite having been born in the pre-computer 1950s I can now apply online to get a duplicate birth certificate from the county where I was born. So what exactly would I put in my SDB were I to have one? Priceless jewels we own but never wear?
FTR, here’s a thread I started a dozen years ago (!) asking about the utility of SDBs. The response was mixed. I see nothing about the social and commercial changes of the last 12 years that make SDBs more essential for ordinary folks. In fact far less.